The various types of corn (i.e., maize) have widely differing uses because of, inter alia, the widely differing carbohydrate content of the kernels. For example, field corn can be a good livestock feed but is not well suited for human consumption, except as the end result of further processing. Sweet corn, on the other hand, can be highly palatable to humans in the raw state. Sweet corn is more desirable as a vegetable corn because of its relatively higher sugar content, higher water soluble polysaccharides (WSP), lower starches and more tender pericarp in addition to other factors that contribute to "quality" (i.e., appearance both before and after removing the husks). It is the WSP which gives sweet corn kernels their smooth, creamy characteristics which contrast sharply with the gritty, starchy effect experienced when eating field corn. Compared to field corn, elevated sugar levels and WSP as well as reduced starch are the factors that most Americans associate with sweet corn. Supersweets are low in WSP but are not, as a group, particularly starchy.
In the industrial practices of processing vegetables (e.g., canning and freezing), both the WSP level and the sugar level can be important. In a typical canning process, for example, the kernels are removed from the ears of sweet corn and the resulting mass of detached kernels (or "cut") is automatically packaged in cans along with a watery liquor. Most commonly, extraneous natural sweeteners (e.g., sucrose or honey) are added to the liquor to increase the natural sweetness of the sweet corn. It is the experience of most canneries that the normal or natural level of sweetness in sweet corn is not high enough for canning. Therefore, assuming equivalent yield, a hybrid corn which was very high in sugars would be an improvement over normal sweet corn and would facilitate the operations of the canning industry. Also, there may be marketing advantages in a "no additive" canned product. Sugar cannot be added to a frozen whole-ear pack.
In most breeding programs to date, development of supersweet hybrids with high sugar content (e.g., sucrose) has resulted in a product which is below normal sweet corn in WSP content. Some of the more interesting studies to produce high sugar content corn have been carried out with a mutant gene shrunken-2 (sh.sub.2). When the genome is homozygous sh.sub.2 sh.sub.2, the sweetness of the corn is increased and the WSP content is often lowered, but it is felt that the higher sugar levels compensate for the lowered WSP levels. In addition, the endosperm weight and starch levels are reduced, resulting in a light weight, easily damaged seed. Germination of these light weight seeds can be a problem both in inbred production and in hybrid strains. To improve vigor and germination, dent corn (a type of field corn) has been used as the background for the sh.sub.2 gene. However, the dominant dent corn genes can necessitate isolation of supersweet hybrids from both field corn and "sugary" sweet corn since cross pollination by any foreign pollen can cause all the kernels to be dent corn in character.
For a review of the effects of genetic interactions of the sh.sub.2 gene with other genes (e.g., the sh.sub.1 gene) with particular emphasis on investigating carbohydrate composition of the kernels, see: Rosenbrook and Andrews (1971) Crop Science 11:536-538; Holder, Glover and Shannon, (1974) Crop Science 14:647-648 and 14:643-646.
A previous disclosure (U.S. Pat. No. 3,971,161 issued July 27, 1976) has described a particular genetic combination using su.sub.1 su.sub.1 sh.sub.2 sh.sub.2. This patent has further described results of some prior studies of the sugary-shrunken genotype. As stated therein, an "apparently unpublished" report detailed some work done on so-called "Ultrasweet" or "Supersweet" sweet corn which was based on the combination of the recessive gene shrunken-2 (sh.sub.2) on chromosome 3 with the dominant starchy-1 (Su.sub.1) on chromosome 4. The shrunken-sugary combination (sh.sub.2 sh.sub.2 su.sub.1 su.sub.1) is also discussed. The sh.sub.2 sh.sub.2 su.sub.1 su.sub.1 combination was said to be "defective to the point of being nearly lethal and impossible to produce except in a background of pseudo starchy-sugary". It was further reported that the combination of sh.sub.2 sh.sub.2 su.sub.1 su.sub.1 can be commercially produced by having the seventh chromosome of Tripsacum (Tr.sub.7) which carries the Su.sub.1 allele, "present as an extra pair in the seed parent". The report then goes on to describe a hybrid produced by the cross sh.sub.2 sh.sub.2 su.sub.1 su.sub.1 Tr.sub.7 .times.sh.sub.2 sh.sub.2 su.sub.1 su.sub.1. Thus, in spite of the exceptionally poor seed quality of the homozygous sh.sub.2 sh.sub.2 su.sub.1 su.sub.1 (see above), the above quoted report suggests using su.sub.1 and sh.sub.2 in a homozygous condition. However, it has not been commercially feasible to employ homozygous sugary-shrunken strains on both sides of a cross.
However, production of a hybrid sweet corn has been achieved by use of su.sub.1 and sh.sub.2 in a particular genetic combination (U.S. Pat. No. 3,971,161, issued July 27, 1976). The parents for the heterozygous F1 ears of this hybrid sweet corn are Sun Seeds proprietary inbred lines and are not well known. These parents are described in the literature. See, for example, the Crop Science articles cited above.
Although normal sweet corn would be used as either the male or the female parent, it was considered preferable in U.S. Pat. No. 3,971,161 to use normal sweet corn as the female parent. The male parent (i.e., still tasseled parent), was a homozygous sugary-shrunken corn. Therefore, using the notations su.sub.1 su.sub.1 sh.sub.2 sh.sub.2 to represent the sugary-shrunken parent and su.sub.1 su.sub.1 Sh.sub.2 Sh.sub.2 to represent the normal sweet corn parent, the system or method of producing commercial quantities of heterozygous F1 seed can be schematically represented as follows: ##STR1##
The F1 was normal appearing sweet corn seed, but because it was heterozygous for Sh.sub.2 sh.sub.2, planting and field pollination produced F2 ears characterized by a 3:1 segregation of normal kernels to shrunken (sh.sub.2 sh.sub.2) kernels. However, phenotypically all kernels appeared the same. Schematically represented, the genetic composition of the triploid (3N) endosperm of these kernels is:
______________________________________ Genotype Phenotype Ratio ______________________________________ ##STR2## Sugary 1 ##STR3## " 1 ##STR4## " 1 ##STR5## Sugary, shrunken 1 ______________________________________
The homozygous sh.sub.2 kernels increased the average sugar content of the "cut" while the normal sweet corn kernels maintained the average WSP at a high level. When a consumer sampled the F2 kernels, only the effects provided by these averages was tasted. It was not apparent that some kernels were relatively lower in WSP and relatively higher in sucrose while other kernels were relatively lower in sucrose and relatively higher in WSP.
The combined effect of 25% high sugar F2 kernels with 75% high WSP F2 kernels was thus very favorable for processing of the "cut". The average content of sucrose, total sugar (i.e., sucrose+fructose+glucose) and WSP in the F2 "cut" on a dry weight basis was also described (U.S. Pat. No. 3,971,161 issued July 27, 1976).
Published data are available for the carbohydrate content of F2 kernels obtained from normal (field) corn, sweet corn, super-sweet corn with a normal (field) corn background, and super-sweet corn with a sweet corn background (see Table below), as well as the individual F2 genotypes su.sub.1 su.sub.1 su.sub.1 Sh.sub.2 Sh.sub.2 Sh.sub.2, su.sub.1 su.sub.1 su.sub.1 Sh.sub.2 Sh.sub.2 sh.sub.2, su.sub.1 su.sub.1 su.sub.1 Sh.sub.2 sh.sub.2 sh.sub.2 and su.sub.1 su.sub.1 su.sub.1 sh.sub.2 sh.sub.2 sh.sub.2 (hereinafter referred to as the "four genotypes"). The following table summarized averages of this published data.
______________________________________ Various carbohydrates 21 days after pollination (70-76% moisture) as percent dry weight Percent of Dry weight Endosperm Total Phenotype Genotype Sucrose Sugar WSP Starch ______________________________________ Starchy ##STR6## 7.3 10.5 0.55 47.0 Sugary ##STR7## 12.5 18.1 36.6 18.3 Shrunken ##STR8## 26.0 32.7 0.67 17.3 Sugary- ##STR9## 39.8 44.3 7.1 2.8 shrunken Average of 19.3 24.7 29.2 14.4 "four genotypes" in 1:1:1:1 ratio ______________________________________
Various hybrids have been produced by crosses of the above genotypes. In all these hybrids, the expected sugar content (based on genotype) corresponded to the measured sugar content. These findings correspond to the predicted sugar content based upon the F1 genotype and on the Mendelian ratios of genotypes in the F2 seeds, assuming each genotype contributes a characteristic sugar content to the "cut". For example, a representative sample of "cut" produced by the hybrid described in U.S. Pat. No. 3,971,161 had a sucrose content of 24.7% (percent of dry weight), in close agreement with the calculated value of 19.3%. For crosses involving the sugary and shrunken loci or their wild-type alleles, the experimental values for sugar content in prior art hybrids have corresponded closely with the calculated values. We have discovered a surprising exception to this correspondence and describe in the "Detailed Description of the Invention" a novel category of hybrids in which the sugar content is higher than the theoretical expected on the basis of the parent genotypes.